Gas turbine engines include a compressor section for supplying a flow of compressed combustion air, a combustor section for burning fuel in the compressed combustion air, and a turbine section for extracting thermal energy from the combustion air and converting that energy into mechanical energy in the form of a rotating shaft.
Modern high efficiency combustion turbines have firing temperatures that exceed about 1,000° C., and even higher firing temperatures are expected as the demand for more efficient engines continues. Many components that form the “hot gas path” combustor and turbine sections are directly exposed to aggressive hot combustion gasses, for example, the combustor liner, the transition duct between the combustion and turbine sections, and the turbine stationary vanes and rotating blades and surrounding ring segments. In addition to thermal stresses, these and other components are also exposed to mechanical stresses and loads that further wear on the components.
Many of the cobalt and nickel based superalloy materials traditionally used to fabricate the majority of combustion turbine components used in the hot gas path section of the combustion turbine engine are insulated from the hot gas flow by coating the components with a thermal barrier coating (TBC) in order to survive long term operation in this aggressive high temperature combustion environment.
TBCs are highly advanced material systems. These coatings serve to insulate the components from large and prolonged heat loads by utilizing thermally insulating materials which can sustain an appreciable temperature difference between the load bearing alloys and the coating surface. In doing so, these coatings can allow for higher operating temperatures while limiting the thermal exposure of structural components, extending component life by reducing oxidation and thermal fatigue.
TBC systems often consist of four layers: the metal substrate, metallic bond coat, thermally grown oxide, and ceramic topcoat. The ceramic topcoat is typically composed of yttria-stabilized zirconia (YSZ), which is desirable for having very low thermal conductivity while remaining stable at nominal operating temperatures typically seen in applications. TBCs fail (or spill) through various degradation modes that include mechanical rumpling of bond coat during thermal cyclic exposure, accelerated oxidation, hot corrosion, and molten deposit degradation. With the loss of the TBC, the component experiences much higher temperatures and the component life is reduced dramatically.
Considering above factors into account, a fixed schedule may be used to inspect these critical components. It is important to schedule the inspection of these critical components as close to the predicted failure of the components to minimize the time the turbine is taken off-line for inspection. In order to reduce the operational costs, it is important to improve the accuracy of evaluation of remaining life of these parts, including having an accurate estimation of the residual life of thermal barrier coatings upon these parts in order to schedule inspection and/or the repair. Accordingly, there is a need to have the ability to accurately estimate thermal barrier coating performance.